Magneto optic recording media are also known by several other names: thermomagnetic media, beam addressable files, and photo-magnetic memories. All of these terms apply to a storage medium or memory element which responds to radiant energy permitting the use of such energy sources as laser beams for both recording and interrogation. Such media modify the character of an incident polarized light beam so that the modification can be detected by an electronic device such as a photodiode.
This modification is usually a manifestation of either the Faraday effect or the Kerr effect on polarized light. The Faraday effect is the rotation of the polarization plane of polarized light which passes through certain magnetized media. The Kerr effect is the rotation of the plane of polarization of a light beam when it is reflected as at the surface of certain magnetized media.
Magneto optic recording media have several advantages over known magnetic recording media:
1. No contact between the medium and a recording head, thus eliminating a source of wear; PA1 2. Using a pulsed laser beam as the writing means, very high density data storage is possible; and PA1 3. With a protective layer on top of a magneto optic layer, the medium is affected less by dust than magnetic media. PA1 1. The medium is initially in a randomly magnetized state. A domain will herein refer to the smallest stable magnetizable region; although in common usage, a domain is a uniformly magnetized region of any size. A selected area of the medium may be magnetized by exposing it to a continuous energy beam and a small magnetic bias field normal to the surface of the medium. PA1 2. A small magnetic bias field oriented perpendicular to the surface or plane of the medium, but oppositely directed to the magnetic field applied earlier is applied over the entire thin film medium. PA1 3. With the biasing field in place, a light beam from a radiant energy source such as a laser beam is directed toward a selected location or bit on the medium where it causes localized heating of the medium to a temperature at or above the Curie and/or compensation temperature. When the laser beam is removed, the bit cools in the presence of the biasing magnetic field and has its magnetization switched to that direction. The medium, in effect, has a magnetic switching field which is temperature dependent. The magnetic biasing field applied to the irradiated bit selectively switches the bit magnetization, with the bit momentarily near its Curie and/or compensation temperature under the influence of the laser. The momentary temperature rise reduces the bit coercive force.
In magneto optic recording, data is written into a medium having a preferentially directed remanent magnetization by exposing a localized area (spot or bit) on the recording medium to an electromagnetic or other energy source of sufficient intensity to heat the recording medium above its Curie or compensation point temperature and simultaneously biasing the medium with a magnetic field. Preferably, the energy source is a laser which produces a monochromatic output beam. The magnetic field required to reverse the magnetization of the recording medium varies with the temperature to which the recording medium is brought. Generally speaking for a given material, the higher the temperature, the smaller the required magnetic field coercive force.
The write or record operation for both Curie point and compensation point writing is as follows:
In the write operation, the write laser beam is focused to the desired diameter (e.g. 1.0 micrometer) onto the surface of the recording medium by an objective lens.
The memory element or recorded bit is interrogated, or read, nondestructively by passing a low-power (e.g. 1-3 mW) beam of polarized light (e.g. a laser beam) through the bit storage site for a sufficiently short time so as not to heat the medium to change its magnetic state. The read laser beam is normally shaped to a circular cross section by a prism, polarized and focused to the same diameter as the write beam onto the recording medium by a lens. When the read beam has passed through the recorded spot, it is sent through an optical analyzer, and then a detector such as a photodiode, for detection of any change or lack of change in the polarization.
A change in orientation of polarization of the light is caused by the magneto-optical properties of the material in the bit or site. Thus, the Kerr effect, Faraday effect, or a combination of these two, is used to effect the change in the plane of light polarization. The plane of polarization of the transmitted or reflected light beam is rotated through the characteristic rotation angle .theta.. For upward bit magnetization, it rotates .theta. degrees and for downward magnetization -.theta. degrees. The recorded data, usually in digital form represented by logic values of 1 or 0 depending on the direction of bit magnetization, are detected by reading the change in the intensity of light passing through or reflected from the individual bits, the intensity being responsive to the quantity of light which is rotated and the rotation angle.
It was previously believed that the signal-to-noise ratio (SNR) or carrier-to-noise ratio (CNR) of an erasable magneto optic medium is proportional to .theta..times.R.sup.1/2 where .theta. is the angle of rotation and R is the reflectivity of the medium. Presently, the relationship between CNR and the parameters of a fully constructed magneto optic medium is not well understood. The process of optimizing media construction appears to be more complicated than simply optimizing .theta..times.R.sup.1/2.
Forty-five decibels in a 30 kHz band width is generally considered the minimum CNR acceptable for direct read after write (DRAW) media. The speed at which the bits can be interrogated and the reliability with which the data can be read depends upon the magnitude of the magneto optical properties, such as the angle of rotation, and upon the ability of the interrogation system to detect these properties. For purposes of this discussion, the noise floor or noise level is measured at the average noise level.
The main parameters that characterize a magneto optic material are the angle of rotation, the coercive force, the Curie temperature and the compensation point temperature. The medium is generally comprised of a single layer or multiple layer system where at least one of the layers is a thin film metal alloy composition. Binary and ternary compositions are particularly suitable for amorphous metal alloy formation. Suitable examples would be rare earth-transition metal (RE-TM) compositions, such as: gadolinium-cobalt (Gd-Co), gadolinium-iron (Gd-Fe), terbium-iron (Tb-Fe), dysprosium-iron (Dy-Fe), Gd-Tb-Fe, Tb-Dy-Fe, Tb-Fe-Co, terbium-iron-chromium (Tb-Fe-Cr), gadolinium-iron-bismuth (Gd-Fe-Bi), gadolinium-iron-tin (Gd-Fe-Sn), Gd-Fe-Co, Gd-Co-Bi, and Gd-Dy-Fe.
Many of the elements which are suitable for the rare earth-transition metal alloy layer react strongly with oxygen and other elements which may be present in the environment in which the media are used. Furthermore, the substrate upon which the alloy layer is deposited may itself contain impurities which react with the alloy layer. Thus, materials are deposited on one or both sides of the RE-TM thin film to protect it. To be effective, such materials must not themselves react with the rare earth-transition metal layer or any other layer, must offer chemical and physical resistance to degradation by heat, humidity, and corrosive chemicals, and must be transparent at the wavelengths used for reading and writing of data (typically about 8200 or 8300 angstroms for a laser diode, or approximately 6328 angstroms for a helium-neon laser, although other wavelengths may be used). A material is "transparent" for the purposes of this discussion when it absorbs less than about 20 percent of the intensity of an incident light beam at a particular wavelength.
Presently used dielectrics include silicon suboxide (SiO.sub.y, y&lt;2), titanium dioxide, silicon dioxide, cerium oxide, aluminum oxide, and aluminum nitride. Most of these materials contain oxygen, which can react with the rare earth element in the magnetizable layer and thereby degrade media performance. All these materials are dielectrics, i.e., they have very low electrical conductivity. This prevents the use of DC magnetron sputtering to deposit them on the other layers of a complete magneto optic medium. Instead, radio frequency (RF) sputtering, evaporation deposition, or reactive sputtering deposition, can be used.